1. Field of the Invention
The present invention relates to a current differential relay that executes a power-line protection in a protected section of a power system by using a signal transmission line between the both sides of the protected section.
2. Description of the Related Art
To protect a power line of a power system, a pulse code modulation (PCM) current differential relay is arranged at each end (an electric power station such as a substation) of a protected section. Each PCM current differential relay samples a power-line current at a first end at regular intervals, and transmits obtained data of the first end to a second end by using a PCM signal transmission line. The PCM current differential relay then acquires data of a second end, executes ratio differential computation by using each data of the first end and the second end, obtains a differential current, and determines presence or absence of a power-line failure within the protected section. As a result of the determination, if a power-line failure is detected in the protected section, each PCM current differential relay is configured to protect the power line in the protected section by tripping a breaker at the first end and disconnecting the power line in the protected section from the power system.
In such case, timings of sampling a power-line current needs to be simultaneous between a device at the first end and a device at the second end. For this reason, conventionally, various methods have been proposed for synchronizing sampling timings between a device at the first end and a device at the second end.
Japanese Patent Publication No. H5-42209 (FIG. 1) describes a technology of synchronizing sampling timings based on timings of transmitting and receiving a signal at each of the first end device and the second end device. In other words, according to Japanese Patent Publication No. H5-42209 (FIG. 1), a signal is transmitted from the first end device to the second end device at regular intervals synchronized with the sampling timings. The second end device obtains a time difference between a receiving time point of the signal and a sampling timing immediately before the receiving time point, and transmits the time difference to the second end device. The first end device synchronizes sampling timings based on the time difference information and a timing of receiving the time difference information by the first end device.
According to the technology of Japanese Patent Publication No. H5-42209 (FIG. 1), under the assumption that a transmission delay time of a signal from the first end device to the second end device is equal to the transmission delay time of a signal from the second end device to the first end device, synchronization is established.
However, a general PCM transmission device transmits a PCM signal by multiplexing it, so that a transmission delay time is determined in accordance with a multiplexing timing. Moreover, the multiplexing timing is determined in accordance with the start of a transmission by the PCM transmission device, consequently, when a power short break or a channel switching of the PCM transmission device occurs, the transmission delay time changes.
For this reason, to synchronize the sampling timing by using the method according to Japanese Patent Publication No. H5-42209 (FIG. 1), it needs to prepare a PCM transmission device that is specially configured to have a constant transmission delay time, and to make the mutual transmission delay time equal between the first end device and the second end device. Such a PCM transmission device is expensive, and the application of the PCM transmission device to a current differential relay results in a high cost on the system.
To solve such problem, conventionally a technology of synchronizing the sampling timing of a current differential relay by using a time synchronization signal of a global positioning system (GPS) has been proposed.
For example, Japanese Patent Application Laid-open No. 2002-186166 (FIG. 1) describes a technology configured to synchronize sampling by measuring a difference between a pulse of a predetermined cycle received by a GPS receiver (hereinafter, “GPS time-synchronization signal”) and a sampling pulse output by a sampling pulse generating unit, and correcting the sampling pulse output by the sampling pulse generating unit based on the timing difference. According to Japanese Patent Application Laid-open No. 2002-186166 (FIG. 1), to synchronize a sampling pulse to a signal transmitted from a GPS satellite, sampling timings can be synchronized between current differential relays without influence of a transmission delay during a data transmission.
Furthermore, as a countermeasure to an interruption of a GPS time-synchronization signal, Japanese Patent Application Laid-open No. 2002-186166 (FIG. 1) proposes methods, for example, making a differential current close to substantially zero or the value of a differential current before the interruption of the GPS time-synchronization signal, or performing sampling synchronization processing based on a transmission delay time that is measured in advance and a present data-receiving timing.
When synchronizing the sampling timing by using a GPS time-synchronization signal, for example, it needs to prepare a method of securing synchronization of the sampling timing when a GPS time-synchronization signal turns abnormal because a GPS signal cannot be received, for example, due to a receiving error.
According to Japanese Patent Application Laid-open No. 2002-186166 (FIG. 1), to secure synchronization of the sampling timing in such case, it needs to perform processing that is different to a large extent from a case of normally receiving a GPS time-synchronization signal, for example, processing of making a differential current between currents at the both ends substantially zero as described above, processing of making a phase difference between currents at the both ends substantially zero, or processing of correcting the sampling timing based on a transmission delay time that is measured during a period while the GPS time-synchronization signal is normal. As described above, according to Japanese Patent Application Laid-open No. 2002-186166 (FIG. 1), because synchronization processing is substantially different between a normal case and an abnormal case of the GPS time-synchronization signal, there is a problem that processing becomes complicated.
In addition, because current information, such as a differential current and a current phase difference, greatly changes when an internal failure occurs inside a power line, the current information needs to be fixed to current information in a state before the failure occurs in order to avoid influence of the change on a sampling frequency. For this reason, there is a problem that processing becomes complicated and unreliable because it needs to confirm conditions under which no failure is found in the system by using another failure detecting unit.